Method of producing a steel ingot



y 2,8, 1970 B A. DIENER ETAL -3,521,95

' METHOD OF PRODUCING A STEEL INGOT Filed April 26, 1967 BUBBLEFORMATION- m/C/-\RBON ROD UNKILLED OR PARTIALLY KILLED CORE FIG. I

KILLED SOLIDIFIED SKIN LAYER INGOT MOLD V//////// //AI" PLATE CARRIERGAS WITH CARBON-CONTAINING MATERIAL 4 FIG .2 L YARBON MATER|AL illONTAINER OF MATERIAL iECO OSABLE IN AOLT STEEL United States Patent Int.Cl. B22d 27/18 US. Cl. 16455 7 Claims ABSTRACT OF THE DISCLOSURE Amethod of producing a steel ingot free of subcutaneous blowholes andvoids, characterised in that after the formation of a skin layer whichhas solidified in the killed condition, there is added to the stillliquid core of an ingot of killed steel an amount of carbon to producesemikilled or unkilled solidification.

SPECIFICATION This invention relates to manufacture of steel ingots.

It is known to pour steel ingots or slabs in the killed, unkilled, orsemikilled condition, unkilled or semikilled solidification beinggenerally preferred because of the reduction of shrinkage voids orcavities (pipes). The elimination of shrinkage voids reduces top wasteconsiderably and thus leads to a not inconsiderable increase inproduction as compared with steel solidified in the killed state.

In the case of unkilled solidification, however, ingot segregationbecomes an inconvenience because the alloying elements of the steel aredistributed irregularly over the cross section of the ingot. Ingotssolidified in the semikilled state do not have this disadvantage; intheir case the distribution of the iron content, added elements andadded alloying elements is homogeneous over the cross section of theingot while there is relatively little shrinkage voids and little topwaste. On the other hand, ingots solidified in the semikilled conditionhave the disadvantage that a part of the carbon monoxide that formsduring solidification remains in the solidified skin layer of the ingotin the form of small blisters. Such subcutaneous blowholes constitute avery disagreeable defect which has a very detrimental actionparticularly in the further processing of the ingots. The formation ofsubcutaneous blowholes is frequently so serious that a considerableproportion of the skin layer must be removed by scarfing or similarmeasures.

It is known that the carbon monoxide produced during solidificationleads to unkilled or semikilled solidification, depending on the amountformed. To what extent unkilled or semikilled solidification takes placein individual cases depends on the extent to which the steel deviatesfrom its carbon-oxygen equilibrium at a given temperature. In order toachieve semikilled solidification, the carbon and oxygen contents areadjusted either by predeoxidation or by vacuum treatment so that theyare appreciably beneath their respective equilibrium values, while thelatter is nevertheless exceeded by carbon and oxygen segregation in theregion of the solidification front. In the case of killed steels thefree oxygen content of the steel is generally reduced, by addingelements having an afiinity for oxygen, such as aluminium, silicon, andtitanium, to such an extent that carbon-oxygen equilibrium is notachieved during solidification.

It has now been found that a steel will solidify in the killed conditionif its carbon and oxygen content is reduced to low values, for example,by vacuum treatment, and if the steel has a manganese content determinedby ICC said values. Expressed differently, this means that the steelwill solidify in the killed state with a given oxygen and manganesecontent provided its carbon content is below a determined value. Thefollowing conditions have been found for dependence of the carboncontent in respect of killed solidification:

For 0 =20 10 c is -0.0035+0.06

For o =30 10 c) is 0.0055+0.06

For o =40 10 c is 0.007S+0.06

The interdependence between the oxygen, carbon, and manganese contentscan be explained more fully by the following example.

EXAMPLE A steel containing 0.030% of oxygen, 0.42% of manganese, lessthan 0.002% of aluminium, and less than 0.01% of silicon requires acritical carbon content of 0.020% by application of the aboveconditions.

The steel was cast into two ten-ton slabs, one of which solidified inthe completely killed condition with a carbon content of 0.017%, andwhich consequently had shrinkage voids. The slab solidified in thekilled condition, without the addition of deoxidation agents, had aclean surface so that scarfing or chipping, such is frequentlyindispensable with aluminium killed slabs, was not necessary.

The second slab which had a carbon content of 0.022% solidified withoutforming shrinkage voids, although it had numerous subcutaneousblowholes. From this it is clear that the killed or semikilledsolidification behaviour with a determined manganese content is onlyslightly dependent on the oxygen content but highly dependent on thecarbon content of the steel.

Starting on this basis, the problem underlying the invention was toproduce a steel ingot free of both subcutaneous blowholes and shrinkagevoids.

According to this invention, solidification of the ingot is conducted insuch a manner that it solidifies in the semikilled condition in the coreand in the killed condition in the skin Zone.

More specifically, the invention consists in producing an ingot having asemikilled core and a killed skin zone by adding, after pouring, anamount of carbon, which is suificient for semikilled or unkilledsolidification, to the core, which is still in the liquid condition, ofan ingot of a vacuum killed steel, after the formation of a skin layersolidified in the killed condition. In this way, while the respectivedisadvantages are eliminated, the advantages of a steel solidified inthe killed condition are combined with those of a steel solidified inthe unkilled or semikilled condition.

This means that the ingot produced by the method of the invention isfree of both subcutaneous blowholes and voids.

The timing of the addition of carbon to the liquid core of the ingotdepends on the thickness of the skin layer which is necessary for thepurpose of fault-free further processing. Since the thickness of thesolidified skin layer increases continuously from the head to the footof the ingot, the moment at which the carbon is added is advantageouslydetermined on the basis of the thickness of the skin layer at the headof the ingot. The dependence'of the solidification on a time factor isobtained in the case of chill casting approximately from the followingformula:

where d is the thickness of the solidified skin layer in 3 centimetresand t the time in minutes after commencement of solidification. Thus thetiming of the addition of carbon after commencement of solidificationcan be determined in accordance with the equation:

(I being in this case the thickness of the skin layer free fromsubcutaneous blow holes which is required for faultfree furtherprocessing. If, accordingly a layer of, for example 2.5 cm., which isfree from subcutaneous blow holes is required, the carbon must be addedone minute after the commencement of solidification.

The quantity to be added in individual cases is determined from thedifference between the carbon content of the melt which, in accordancewith the above conditions, is required for semikilled or unkilledsolidification, and the actual carbon content. For a slab solidified inthe killed condition, which is described in the above example, anadditional quantity of 0.003% of carbon would ac cordingly be necessary.It is, however, advisable in every case to add to the core of the ingota quantity of carbon which slightly exceeds the amount necessary.Practice has shown that carbon additions of below 0.01% are generallysufficient, this corresponding to the addition of 1 kg. of carbon in thecase of a ten-ton slab. Experiments have shown that the slight increaseof the carbon content of the steel resulting from the addition of carbonin accordance with the invention does not result in any substantialdeterioration of its technological properties. Since the carbon is addedto the liquid core of the ingot for the purpose of preventing theformation of shrinkage voids after solidification of the outer zonewhich is free from subcutaneous blow holes, it is sufiicient for thecarbon content to be increased solely in the region of the top of theingot.

Our inventive method will be best understood from the followingdescription of specific modes thereof when read in connection with theaccompanying drawings, wherein:

FIGS. 1 to 4 are sectional views of an ingot mold showing four diiferentmodes of performing the method of the invention for producing a steelingot free of subcutaneous blow holes and voids.

According to the invention, as shown in FIG. 1, carbon is added to thesteel by immersing carbon rods or rods of a carbon-containing materialin the liquid core of the ingot. However, as shown in FIG. 2, containersholding carbon or carbon-containing material and adapted to dissolve ordecompose in steel may also be introduced into the core of the ingot.Another possible method of introducing carbon or material containingcarbon into the core of the ingot is shown in FIGS. 3 and 4 andcomprises the use of a carrier gas or of carbon-containing gases whichare decomposed in the presence of steel, such as for example, methane.

What is claimed is:

11. A method of producing a steel ingot free of subcutaneous blow holesand voids, which comprises the steps of first forming a steel ingot witha skin layer in killed condition by solidification of a killed liquidsteel, and then adding to the still liquid core of the ingot an amountof carbon to produce solidification ranging from unkilled to partiallykilled solidification.

2. The method according to claim 1, in which the liquid steel issubjected to vacuum to adjust the carbon and oxygen content thereof tocontrol the steel to killed condition prior to pouring the liquid steelinto a mold.

3. The method according to claim 1 in which carbon is introduced byimmersing a rod comprising carbon in the liquid core of the ingot.

4. The method according to claim 1 in which a carbon rod is immersed inthe liquid core of the ingot.

5. The method according to claim 1 in which the carbon is introduced byimmersing a container formed of material decomposable in steel into theliquid core of the ingot, the container containing carbon material.

6. The method according to claim 1 in which carbon is introduced orblown into the liquid core of the ingot by blowing a carrier gas havingcarbon-containing material in suspension therein.

7. The method according to claim 1 in which carbon is introduced byinjecting carbon-containing gases which decompose in steel into theliquid core of the ingot.

References Cited UNITED STATES PATENTS 585,036 6/1897 Hunt 75481,294,209 2/1919 Walker. 2,518,738 8/1950 Woods. 2,853,376 9/1958 Blancet al. 7548 2,837,800 6/1958 Hachiya et a1. 16456 2,952,534 9/1960 Quinnet al. 75-57 X 3,188,198 6/1965 Moore 7557 3,208,117 9/1965 Goedecke etal. 164--56 2,389,576 11/1945 Kinnear l6496 3,410,681 11/1968 Orban16456 X 3,414,041 12/1968 Cutton 16456 3,436,209 4/1969 Lojas 16455 X3,274,681 9/1966 Lohman 164-95 X FOREIGN PATENTS 599,323 10/1959 Italy.325,003 9/ 1920 Germany.

J. SPENCER OVERHOLSER, Primary Examiner V. K. RISING, Assistant ExaminerUS. Cl. X.R.

